Packages of fiber (e.g., fiberglass, carbon, aramid) are manufactured to maximize volume per unit of weight. Although fiberglass will be described herein, the principles of the invention described herein apply to other types of flat fiber tows. Glass strands produced by companies such as Owens Corning, PPG, Saint-Gobain and the like are produced by winding glass strands in a flat band. Thousands of filaments are consolidated at a discharge bushing from a glass furnace and treated, sized, consolidated, and wound on a temporary mandrel at speeds up to 1000 meters per minute or more. The wind profile places these strands in a helical fashion, creating a cylindrical tubular package called a doff. An exemplary doff may have a height of 10 inches, an outside diameter of approximately 11 inches, and an inside diameter of approximately 6.5 inches. Each doff weighs up to 40 lbs.
These doffs are then wrapped with a shrink wrap plastic on the outside and the internal temporary mandrel is removed. With the internal temporary mandrel removed, the package becomes a center-pull doff. The ultimate processor of the composite material must pull the flat strand from the center of the doff. The cylindrical tubular doffs include a vertical axis and center pull of the flat fiber tow is vertical, upwards out of the central space of the doff vacated by the temporary mandrel.
These center-pull doffs are made with different yields of glass fiber. For example, a 675-yield fiberglass strand from PPG means there will be 675 yards per pound of fiberglass. A 113 yield from Owens Corning will have 113 yards per pound of fiberglass. There are many types of yields produced. In the manufacture of these various yields, there are a myriad of helical patterns that have been developed by the manufacturers for automatic winding of the flat strands of fiber as they exit the glass furnace. A 675-yield doff from PPG has approximately 4.2 winds per helical cycle. This means that there are 4.2 turns of the manufacturer's temporary mandrel for one helical cycle of the flat strand of fiber. One helical cycle runs from the bottom of the doff to the top of the doff (or the top of the doff to the bottom of the doff). A 113 yield doff from Owens Corning has approximately 2.05 winds per helical cycle. This means that there are 2.05 turns of the manufacturer's temporary mandrel for one helical cycle of the flat strand of fiber. The wrap patterns of the doffs have been developed to optimize the size, shape, and density of the doffs.
Naturally, when one full circumferential pull-out of fiberglass ribbon is pulled from the center of the top of a doff, a 360-degree “turn” or “twist” occurs in the fiberglass ribbon.
In a 675-yield doff, there is a total distance of approximately 80,000 feet of fiberglass ribbon or tow, and about 40,000 helical wraps of the flat ribbon. This means that a fabricator pulling the tow or ribbon from the center of the doff will have 40,000 turns (or twists) of the ribbon over the entire doff.
Some types of processing (e.g., filament winding, tape laying) require that 100% of these turns (or twists) be removed as the fiber tow or ribbon is pulled out. Glass manufacturers repackage doffs onto tangent-pull spools so that downstream processing can have continuous flat ribbons; but this can cost an additional 5 cents per pound over a center-pull doff. Other processing methods (e.g., pultrusion, knitting, weaving) simply live with the flat ribbon turning in the longitudinal direction and the results of the turns/twists (e.g., an inefficient composite lay-up because of the greater thickness and bulk with a turned ribbon or tow). Maintaining tows flat and unturned is advantageous for all composite processing.
U.S. Pat. No. 6,581,257 to Burton, et al. (“Burton”) attempts to achieve flat and unturned tows. In Burton, doffs are laid horizontally on their side (i.e., longitudinal axes of doffs are horizontal). The doffs are rotated using a clamped doff via an outside diameter spoked mechanism. Several doffs are integrated into a belt system such that a series of doffs are rotated at the same speed, attempting to match rotational speed to tow pull-out speed.
Burton requires the roll-up of the fiberglass onto a beam (for later and subsequent processing), which adds time and expense to the process. Burton also requires precise speed control of the beam and the doff, but does not elaborate on how the rotational speed of the doff is calculated or adjusted. The helical pattern on the wrapping of the doff creates a variable distance per revolution as well as a significant distance variation per revolution due to inside diameters changing constantly and significantly from a full doff to an empty doff. To precisely take out all 40,000 turns of a 675-yield doff by trying to match the speeds would be impossible with Burton's disclosed method. This is especially impossible when performed simultaneously with twenty five (25) doffs as shown in FIG. 2 of Burton. The length of a tow in one revolution of a 675-yield package increases by 0.0003532 inches as the 40,000 turns are removed from a 6.5 inch inside diameter at the beginning of a new doff to the 11 inch diameter at the end of the doff. Burton does not disclose how to make a speed variation in doff rotation that can accurately reflect such minute changes in length in tow length. Additionally, as the helical wind reaches the doff-top, it changes helical angle abruptly and returns in the opposite direction, resulting in a speed-discontinuity. Burton does not address this speed-discontinuity issue. Furthermore, Burton admits that the twist removal is only an average “over an extended length of yarn or strand” so Burton does not remove 100% of the twists or turns in the tow.